07 Load-bearing capacity of geotextile bags for anchors in avalanche and rockfall protection

Witek Max, Lifa Imad, Braun Seraina, Glover James, Röschmann Nadia

  1. Fachhochschule Graubünden, Institut für Bauen im alpinen Raum (IBAR), Chur, Switzerland

Non-tensioned ground anchors are the standard foundation method in avalanche and rockfall protection. Anchors are inserted into boreholes with an injection pipe and filled with mortar. Mortar loss in densely packed gravel sands or non-fissured rock is low. In fissured rock or in highly permeable soils, mortar can leak into surrounding cavities causing costly losses. To reduce mortar loss geotextile grouting bags are inserted with the anchors. Currently, only a few types of geotextile bags are in use in Switzerland, and little is known of the different installation techniques to achieve correct load bearing capacities for anchors.

From practical experience applying anchors with geotextile bags, the achieved load bearing capacities deliver very mixed results. Some anchors achieve satisfactory load-bearing capacities and some fail at a fraction of the nominal load. There is no explanation for this behavior yet. The bonding of the bags with the surrounding ground depends on the porosity of the bags and their elastic capacity to fill gaps and cavities. The different types of geotextile available govern these properties, additionally the interaction between mortar and bag is also of importance.

To investigate how the load bearing capacity of anchors are affected by the use of different geotextile bags, field tests are being carried out. Test anchors of the same length and bore hole diameter, with different geotextile bags are installed in highly permeable terrain and allowed to cure. The anchors are then pulled with an anchor pulling apparatus to measure the maximum bearing load of the anchors.

Additionally, new large-scale controlled laboratory tests are designed to supplement standard tests of the geotextiles for use as anchor bags. The standardized tests form part of new construction guidelines for applying anchors with geotextile bags for anchors. The tests for product approval and the installation conditions form a new guideline.